Method of recovering gold from saprolite gold ores and other refractory slow settling gold ores



charcoal to form a concentrate.

Patented Jan. 1940 PATENT v or F1 01:

salmon or nscovnamo com .m'ou suaom'ra com OBES AND o'rlma as- I mc'roax sLow smrnnva GOLD onus Bruce D. Crawford, Grass Valley, 0eiif., asaignor to American Cyanamid Company, ,N. Y., a corporation of Maine,-

Ne me No Application November 17, 1087. i I

Serial Nit-175,0

lGlaim. (c1. 75-z This invention relates to the recovery of gold from clayey saprolite ores.

The saprolite-ores, which cover ,a considerable area in the Piedmont region of the southern United States, contain large amounts of gold but this type of ore has hitherto not proven amenable to any of the ordinary methods of concentration used on precious metal ores. The ore is a claylike material which generally retains the structure of the rock from which it is derived. The extremely slimy material can not be treated by the ordinary processes of cyanidation or flotation because of the slimlness and any attempts to deslime the ore result in the loss of a large proportion of the gold. For this reason the saprolite gold ores, although known for the last forty years, have not been generally exploited for the recovery of gold. I

According to the present invention, I have found that a process which has been applied to precious metal oresof a different character solves the problem of recovering gold from the saprolite ores with high efliciency. This procedure essentially consists in cyaniding, precipitating the gold with charcoal and then floating the auriferous Despite the fact that the gold in saprolite ore cannot be float-, ed, apparently the auriferous charcoal is of such a nature that can be separated from the clay slimes of the ore by a simple flotation procedure. I do not know just what the reason is why the gold changes in its nature so that although not recoverable by flotation in the original ore, a high recovery is obtained in the form of auriferous charcoal. The present invention is therefore not limited, to any particular theory of action of the process.

The invention is applicable to any carbon which gives efllcient precipitation in cyanide treatment. I have found, however, that contrary to the published patent literature, ordinary high tem- ,perature pine charcoal gives as good and even better results than activated carbon. I have not determined why in the present process cheap pine charcoal or Australian charcoal give as good or better results than specially activated carbon which sells for many times their price. While the present invention is therefore not limited to the use of hightemperature charcoal and in its broader aspects includes the use of activated carbon, in a more specific aspect, the extraordinary effectiveness of cheap high temperature charcoal in the present process is an important feature of the invention. Low temperature charcoal, for example, charcoal prepared at 800 1''. does not appear to be effective and I therefore ,prefer to use high temperature charcoal produced at temperatures around 800' C. and higher. There is no sharp dividing line, however,'and charcoal produced at slightly lower temperatures still exhibits considerable activity.

It is an advantage of the present invention that the charcoal, if it has lost its precipitating power, can be brought back by reheating, for example to about 1000 C.- This is an important advantage as it permits the reuse of charcoal which has lost its effectiveness.

The charcoal may be added at the beginning of the cyanidation, during the cyanidation or at the end with little effect on the gold recovery. It is an advantage of the present invention that the point of addition of the charcoal is not at all critical. I

The invention will be described in conjunction with the specific examples which are typical applications of the invention to typical saprolite ores. It should be understood that recovery wi vary somewhat with diflerent ores.

I Example 1 A saprolite. gold ore containing 0.065 oz. of gold per ton was crused and then cyanided for 20 hours in a solution containing crude calcium cyanide having 0.053% sodium cyanide equivalent and 0.110% calciumoxide equivalent. The pulp density was 25% by weight of solids. After cyanidation, the pulp was agitated for 15 minutes with 2 lbs./ton of pine charcoal prepared by heating in a crucible at 825 C. The auriferous charcoal was then recovered by floating the pulp in.

a Fagergren flotation machine using 0.1 lb;/ton of a :50 mixture of sodium diethyland disecondarybutyldithiophosphates, 0.5 lb. per ton cresylic acid and 0.5 lb./ton of pine ofl. The metallurgical results are as follows:

Percent Percent Oz. ton w... 1.. 35m- Example 2 A saprolite ore containing 0.0998 02. gold/ton was cyanided for 24 hours at a pulp density of 33% solids by weight, using a cyanide solution as described in Example 1. The pulp was then agitated for 15 minutes with 6 lbs./ton of pine charooalandwasthenfloatedinaFlers machine using 0.31b./ton o! kerosene and 0.82

lb./ton o! a i'rother consisting of a mixture of paramnalcoholsoi'ltomcarbonatomsan'da saturated hydrocarbon which is sold by the American Cyanamid Company under the name or "in-other 40". as iollows:

A sample of saprolite ore was cyanided for 25 hours at a pulp density of 25% solids by weight, using a cyanide solution as described in Example 1. The cyanided pulp was then agitated for 15 minutes with 1.5 lbs/ton of charcoal which had been preheated to 1000 C. for one hour and allowed to cool in a loosely covered container. The pulp was then floated in a Fagergren machine with 0.45 lb./ton o! kerosene and 0.19 lb./ton of the irother described in Example 2. The metallurgical results are as follows:

Percent Percent OsJton Product 18m Au fiztgiki r Iced 100. 00 0. 0667 100. 00 l-xotstion concentrate. 2. 8132 96. F iotation tail. 0. (X126 4. 40 W sste solution 0. 00005 0. 01

It will be apparent that preheating the charcoal has a marked effect and preheated charcoal is more eilective than charcoal which has not been preheated. Tests were made with the ore of Example 3 with diirerent 'charcoals; namely,

The. metallurgical results are pinecharooahpreparedbyheatingtom' 6., Australian charcoal preheated to 1000' (2., and activated carbon. The recoveries at 0.5 lbJton oicharcoalwereinthew'a-beingiustW for the Australian charcoal, 85% for acti- I vsted carbon and about 8'1 ior pine charcoal. At about 1lb./ton of charcoal. the recoveries were about 97%, the preheated Australiancharcoal showing slightly higher recoveries, the pine charcoal a little less and the activated carbon the 10 least. The figures were 98.9% for preheated Australian charcoal, 97.79% for pine charcoal and 97.21% for activatedcarbon. It will be apparent that in spite of its activation the much more expensive activated charcoal was not better ll than cheap pine charcoal. In tact, the tests indicate that the three materials ran so close together that for all practical purposes they may be considered as having the" same efliciency. It is evident, therefore, that in the present process, I the charcoal is not behaving in the same way as in the cases where activated carbon has been said to give improved results. This is a peculiar situation in the present process and one, the reason for which I have not as yet been able to .8 determine. It is, however, a great economic advantage of the present process because pin'e charcoal costs only about. y, as much as activated carbon.

In the claim, the expression "high tempera- 00 ture charcoal will be used to cover charcoals which have been produced at temperatures 0! about 800 C. or higher. It should be understood that the expression will have no other meaning. I

What I claim is: '80 A method of recovering gold from auriferous saprolite ores which comprises cyaniding a pulp oi the ore, treating the cyanided pulp with charcoal which has been preheated to about 1000 C.

and subjecting the mixture to a froth notation process in the presence of a reagent favorable to the flotation or auriterous carbon.-

BRUCE D. CRAWFORD. 

